Proguard: avoiding naming collisions with pre-obfuscated library JARs

From the progaurd manual,

If an input jar and a library jar contain classes in the same package, the obfuscated output jar may contain class names that overlap with class names in the library jar. This is most likely if the library jar has been obfuscated before, as it will then probably contain classes named 'a', 'b', etc. Packages should therefore never be split across input jars and library jars.

So it looks like using your own package is the recommended answer.